Use `WeakMap` to store `MessageChannel`

We use `Window` objects as keys on a map. Windows can be removed.
`WeakMap` has the characteristic that keys that are claimed by the
garbage collector are removed from the map. In this way, we are memory
friendly and don't retain any defunct object.

src/shared/port-provider.js

@@ -70,8 +70,10 @@ export class PortProvider {
     // Although some channels (v.gr. `notebook-sidebar`) have only one
     // `MessageChannel`, other channels (v.gr. `guest-sidebar`) can have multiple
     // `MessageChannel`s. The `Window` refers to the frame that sends the initial
-    // request that triggers creation of a channel.
-    /** @type {Map<Channel, Map<Window, MessageChannel>>} */
+    // request that triggers the creation of a channel. We use `WeakMap` so that
+    // entries are removed from the map when the garbage collector reclaims the
+    // removed window.
+    /** @type {Map<Channel, WeakMap<Window, MessageChannel>>} */
     this._channels = new Map();
 
     // Two important characteristics of `MessagePort`:
@@ -214,7 +216,7 @@ export class PortProvider {
 
       let windowChannelMap = this._channels.get(channel);
       if (!windowChannelMap) {
-        windowChannelMap = new Map();
+        windowChannelMap = new WeakMap();
         this._channels.set(channel, windowChannelMap);
       }